Revamp usrsw to use new exti code.
Add user switch callback function.

Stack layout is different when -g used, so need to handle the pendsv
jump differently.  Addresses Issue #315.

This commit also introduces board directories and moves board
specific config into the appropriate board directory.

boards/stm32f4xx-af.csv was extracted from the STM32F4xx datasheet
and hand-tweaked.

make-pins.py takes boards/stm32f4xx-af.csv, boards/stm32f4xx-prefix.c,
and boards/BOARD-NAME/pins.csv as input and generates the file
build/pins_BOARD_NAME.c

The generated pin file for PYBOARD4 looks like this:
https://gist.github.com/dhylands/9063231

The generated pins file includes all of the supported alternate
functions, and includes upsupported alternate functions as comments.

See the commnet block at the top of stm/pin_map.c for details on
how to use the pin mapper.

I also went ahead and modified stm/gpio.c to use the pin mapper.

To follow up pull request #309.

GPIO functions are crude, and need to be replaced with better ones!

Raw REPL mode is activated by CTRL-A, and allows you to more easily
communicate with the pyboard over the serial channel.  In particular,
automated communication using, eg, pyserial on the PC side.

Ultimately all static strings should be qstr.  This entry in the type
structure is only used for printing error messages (to tell the type of
the bad argument), and printing objects that don't supply a .print method.

If SD card is present on (soft) reset then it's mounted on 1:/ and files
can be openned using, eg, open('1:/test.txt', 'r'), or 'w' for writing.

Using PendSV interrupt at lowest priority, code can now raise an
exception during an interrupt by calling pendsv_nlr_jump.  The exception
will be raised when all interrupts are finished.  This is used to trap
ctrl-C from the USB VCP to break out of running Python code.

To partly address Issue #220.

* Move RTC code from main to a separate module.

f2d and d2f functions from libgcc does not work correctly, most likely
due to the ABI being incorrect.  libgcc disabled for now.

mpconfigport.h is now included by stm32f4xx.h so that all STM files have
access to these defines.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

* Add the option to pass OSC frequency on command line.

CMSIS V3.01 -> V3.20. Now in stm/cmsis to keep separate from ST
libraries.

Thanks to Dave Hylands for this.

* Add simple ADC driver, with support for ADC1 and all channels.
* Export MicroPython ADC object with read_channel function.
* Add stm32f4xx_adc.c and adc.c to Makefile.

See pyb_usart_global_debug variable.

Also did some work on USB OTG, but nothing working yet.

* Add a DEBUG flag to Makefile, enables: "-O0 -ggdb"

Use make V=1e make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.

This should fix issue #117

While updating the teensy Makfile, I noticed that the stm Makefile
was using calling arm-none-eabi-objcopy and size rather than
$(OBJCOPY) and $(SIZE), so I added these 2 for consistency.

Updated mconfigport.h to enable GC

* Add a TARGET definition to Makefile (default PYBOARD).
* Add support for discovery LEDs in led module.
* Add support for discovery user switch in usersw
* Add EXTI interrupt handler for discovery user switch on (PA0).
* Parameterize led and usrsw modules pins and port.
* Issue #83